Modern construction techniques make building far more airtight and insulated than they used to be. This is both good and bad. One the one hand, new homes are way more energy efficient than they used to be. On the other, interior air can get stuffy and stale. Use a recovery ventilation unit to bring fresh air into the system to fix this problem. A good recovery ventilation unit will keep interior air fresh without bringing in unconditioned, allergen-infested air at at the same time.